The Discovery of the Electron
According to the ancient doctrines of Democritus and Epicurus, atoms are tiny indivisible masses rattling about through the vacuum of space. This atomic hypothesis was preserved—in one form or another—for over 2000 years. Yet by the close of the nineteenth century it was still not universally accepted. The most convincing evidence for atomism was rather indirect, having come out of (i) the study of chemical reactions, (ii) the kinetic theory of gases and (iii) Boltzmann’s probabilistic interpretation of entropy. Even among the atomists there was considerable disagreement regarding the nature and structure of the atom itself. For example, the followers of Roger Boscovich believed atoms to be little more than point-like mathematical centers of force. Boscovich’s atoms possessed an unchangeable mass, they could move through the vacuum of space, and they were endowed with an irreducible power to attract or repel other atoms. A very different view of the atom was maintained byWilliam Thomson (Lord Kelvin). Inspired by the earlier work of Hermann von Helmholtz, Kelvin imagined atoms to be like minuscule smoke-rings traveling through an all-pervasive frictionless fluid medium—the æther. These so-called vortex ring atoms were indivisible local excitations which could emit light by vibrating like the rim of a tiny ringing bell. In this way, the vortex ring model of the atom offered the possibility of explaining the complicated emission spectra of atomic gasses.
Kelvin’s vortex theory of the atom aroused the interest of another Thomson, J.J., who published his Treatise on the motion of vortex rings in 1883 while studying at the University of Cambridge. A few years later, J.J. Thomson would propose the existence of subatomic particles which weighed considerably less than the smallest known atom. In the reading selection below, Thomson describes his famous 1897 discovery of subatomic electrically charged particles. It is taken from the first chapter of his 1907 book on The Corpuscular Theory of Matter.